Innate and adaptive immunity constitute two distinct but functionally interrelated systems that collectively effect host responses to bacterial, viral and other pathogens. Further understanding the relative roles of these two major arms of immunity is confounded by their genetic complexity, interconnectivity and inherent limitations of studying early immunity in mammalian model systems. Zebrafish is a well-characterized genetic model system in which development is ex utero. Integral components of its immune system are structurally and functionally orthologous to those found in mammals. Unlike mammalian developmental immunology models, a temporal distinction exists in zebrafish in which a protracted period of innate immunity precedes the functional rearrangement of immunoglobulin (Ig) and T cell antigen receptor (TCR) genes and lymphocyte development, allowing the study of innate immune responses in relative isolation. In zebrafish as well as in humans and other mammals, complex families of Ig superfamily (IgSF) members are expressed on cells that mediate immune functions. Several such large families of diversified molecules, termed immune domain-containing receptors (IDCRs), have been identified in zebrafish and two families of these molecules are the focus of these investigations. The genomics of IDCRs will be characterized, transcripts will be recovered, their functional domains will be expressed, ligand-binding properties determined and the functional relevance of IDCRs to immune response evaluated. In order to accomplish these goals, a variety of platforms will be developed for functional characterization, including several approaches for detecting low affinity interactions. The continuing refinement of the zebrafish genome affords the possibility to target and genetically disrupt receptor functions using: 1) morpholino (including caged analogs)- based down regulation of individual receptors and 2) targeted disruption of specific genes through an efficient site-specific zinc finger nuclease-based enzymatic method, which affords rapid selection of recombination events. The roles of IgSF-related molecules in the innate immune response in early development will be studied using in vivo bacterial and viral disease-challenge models. The ubiquitous presence of large, diversified families of IgSF receptors throughout the vertebrates makes it likely that insight will be gained into how corresponding molecules function in immune protection in humans. PUBLIC HEALTH RELEVANCE: Complex families of genes that encode immunoglobulin-related receptors are involved in a wide range of functions that relate to recognition of pathogens and regulation of cooperative interactions of the innate and adaptive immune systems. The zebrafish represents a powerful emerging model for understanding the genetic basis of complex developmental processes and a large number of diseases that also are seen in humans. In zebrafish, innate and adaptive immunity are transiently compartmentalized during development. Large families of immunoglobulin gene superfamily (IgSF) genes are present in zebrafish that resemble those seen in other vertebrates, including man, where they are thought to play a central role in the modulation of immune responses. The function of these genes is not understood despite significant interest and effort. The functions of select members of several different families of IgSF genes will be characterized at the genomic level, their products will be expressed and potential ligands will be identified. The role of specific genes in different pathogen challenge models will be assessed during early development utilizing morpholino and new gene disruption technologies will be used to access gene functions throughout development into the adult stage. Collectively, the studies will afford new insight into how the products of several different gene loci encoding immune-type molecules influence host response(s) to pathogen challenge, thereby providing basic insight into mechanisms of disease susceptibility and resistance.